These are devices designed to allow an individual astronaut to maneuver around in space freely while unattached to any spacecraft or station. They usually work on the principle of expulsion of compressed gasses, usually drawing from one or more canister of inert gas separate from the spacesuit’s life support system. Because these units are designed much more for precision maneuvering in a relatively small volume and less for extended excursions from one point to another in orbit, more powerful means of propulsion like full on chemical rockets are usually deemed excessive, in terms of volume, weight, complication, and control.

However, even relatively ‘simple’ systems like compressed gas expulsion can be greatly complicated by free fall conditions. How bodies move and behave in microgravity does not come instinctively to many people, and extensive training is usually required to use any of these devices properly lest one is sent tumbling or spinning out of control.

Because of today’s safety-above-all environment for manned space flight, individual maneuvering units are used today only as emergency back-ups, and astronauts on EVA are always tethered or attached to a larger spacecraft or station if an excursion is called for. However, in the future this may not always be the case. Larger construction or repair projects in orbit may make even long tethers impractical for efficient work conditions. Exploration of microgravity bodies such as asteroids may also need to eschew tethers. Plus, if military options involving infantry are ever carried out in space, tethers would obviously be a liability.

GAS GUNS
Tech Level: 9

These were developed for the Gemini Program, more formally called the Hand-Held Maneuvering Units (HHMU) but also known as zip guns or maneuvering guns. They were deployed on the Gemini 4 and 10 missions during space walks. In science fiction, a more advanced and long-enduring version was used by Reed Richards of The Fantastic Four while exploring the world-ship of Galactus.

The real-life HHMU consisted of a gas canister of pressurized oxygen connected to an external expulsion nozzle and an oversized, simplified trigger assembly that would allow its easy handling and use by spacesuited astronauts. It was deigned only for use around the Gemini capsule, and did not have capacity for extended use.

The fictional type of gas gun used Reed Richards had an umbilical to his spacesuit’s backpack, where presumably it was fed by a much larger gas canister than the real-life HHMU had. If this technology had been further developed beyond the Gemini program, this seemed like a logical progression for such devices, to allow for both a greater delta-v and endurance.

Theoretically, such gas guns are the simplest type of maneuvering unit for an astronaut; you basically just aim along the axis of your desired trajectory and shoot. However, in microgavity, one has to precisely aim the gas stream through the astronaut’s center of gravity. Otherwise, the user may begin tumbling and the trajectory will be off. On Gemini 4, one of the astronauts found the gun useful, while the other hated it. Like with most other maneuvering units, extensive training would be needed to use these effectively.

While gas guns have fallen out of favor for more sophisticated maneuvering technology, they’re also the easiest type of unit that can be kitbashed together in an emergency or when resources are otherwise low. Integrated computer targeting and navigation systems may allow the user to more precisely aim the gun for maximum efficiency.

BACKPACK MANEUVERING UNITS
Tech Level: 10

Designs for these date back to the 1960s, but were not actually put into use in space until 1984. Both the US and the USSR developed and used backpack maneuvering units before retiring them in light of more stringently safety-oriented EVA procedures.

The first such device put into practical use was the Manned Maneuvering Unit (MMU), employed on three different Space Shuttle missions in 1984. The MMU was designed to fit over the standard portable life support system backpack used on Shuttle EVAs. The MMU used 24 nitrogen gas thrusters that allowed the astronaut to perform a wide range of maneuvers along all three axes. Rotational and translational hand controllers found on the ends of fold-down arms allowed the astronaut precision control. The unit carried enough 5.9 kilograms of gas, enough to serve the astronaut for over 6 hours of EVA maneuvering with judicious use.

The MMU could be replenished from the Shuttle’s own nitrogen supply, and it ran on a parallel redundancy system as a safety feature. The unit massed over 146 kg; an astronaut likely would not even be able to stand up with the unit attached while on Earth.

Though the MMU is retired, it has been replaced with a much smaller, simplified version called SAFER (Simplified Aid For EVA Rescue). The SAFER unit is worn on all current spacewalks and is intended for emergency use only.

A Soviet version of the MMU called the SPK was tested on Mir in 1990. However, they (and their successors in the Russian Space Agency) decided to go with more safety-conscious methods such as tethers and astronaut-anchoring cranes, and the SPK’s development was discontinued.

In the future, as construction and work in orbit and beyond greatly expands, backpack maneuvering units may come back into vogue. These would likely be much more compact and lighter than the MMU, closer in size and mass to the SAFER unit but perhaps with larger gas tanks attached to allow for much more extended use.

Advanced versions of a backpack maneuvering unit may also have autonomous or semi-autonomous computer controls, allowing the system to be activated and controlled remotely if need be, in case the astronaut is incapacitated in some way.

SUIT-INTEGRATED MANEUVERING UNIT
Tech Level: 12

These were used most prominently in the hard science fiction anime series Planetes, but the technology is occasionally seen in other scifi sources. Instead of a separate backpack unit, the maneuvering jets are integrated into the suit itself, with the propellant gas tank carried atop the suit’s PLSS backpack.

In the Planetes version, the exact gas used is never really specified, but presumably it’s a non-reactant gas like nitrogen that real maneuvering units use. Adjustable nozzles are located around the suit, mainly in the forearms, calves, boots, and torso. Intricate finger controls allow precision control over the system, and the system could be activated and flown remotely if needed.

Fewer nozzles were needed than with the MMU as its assumed that the astronaut would be able to move and twist his limbs as needed to use the jets to maximum effectiveness. Though this type of system would need even more extensive training than the MMU to use effectively, it would ultimately allow for more precise and intricate maneuvers than any backpack unit.

An interesting feature the Planetes system is that it had an automated orientation system. If the astronaut found himself disoriented, in an uncontrolled tumble, or in some other emergency, he’d activate the suit’s self-orientation system. Alternately, it could be activated remotely by the crew of his ship or mission control. The suit’s computer system would then take over, stop the tumble, and orient and stabilize the astronaut toward a set or predetermined coordinates. Since the Planetes suits were meant for use exclusively in Earth orbit, the suit would orient the astronaut facing Earth.